Different measurement principles are known for the determination of the flow speed or of the throughflow on an ultrasound basis. In a Doppler process, the frequency shift of an ultrasonic signal reflected at the flowing fluid that differs in dependence on the flow speed is evaluated. In a differential time of flight process, a pair of ultrasonic transducers is mounted with mutual offset in the longitudinal direction at the outer periphery of the conduit, said pair of ultrasonic transducers transmitting and registering ultrasonic signals alternatingly transversely to the flow along the measurement path spanned between the ultrasonic transducers. The ultrasonic signals transported through the fluid are accelerated or decelerated by the flow depending on the running direction. The resulting time of flight difference is calculated using geometrical parameters to form a mean flow speed of the fluid. The volume flow or throughflow results from this with the cross-sectional area. For more exact measurements, a plurality of measurement paths each having a pair of ultrasonic transducers can also be provided to detect a flow cross-section at more than one point.
The ultrasonic transducers used to generate the ultrasound have an oscillating body, frequently a ceramic material. An electric signal is, for example, converted into ultrasound, and vice versa, with its aid on the basis of the piezoelectric effect. Depending on the application, the ultrasonic transducer works as a sound source, as a sound detector or as both. In this respect, a coupling has to be provided between the fluid and the ultrasonic transducer. A widespread solution comprises allowing the ultrasonic transducer to project into the conduit with a direct contact to the fluid. Such intrusive probes can make exact measurements more difficult due to a disturbance of the flow. Conversely, the immersing ultrasonic transducers are exposed to the fluid and to its pressure and temperature and are thereby possibly damaged or lose their function due to depositions.
Techniques are generally also known in which the inner wall remains completely closed. One example is the so-called clamp-in assembly, for instance in accordance with U.S. Pat. No. 4,467,659 by which ultrasonic transducers are fastened to the conduit from the outside. However, only diametrical measurement paths can thus be implemented through the conduit axis, whereby additional errors are generated with non-axially symmetrical flow profiles.
U.S. Pat. No. 6,895,823, which corresponds to EP 1 378 727 B1, proposes attaching the ultrasound-generating elements to an outer side of a wall. Unlike the clamp-on technique, the ultrasonic transducer is in this respect so-to-say integrated into the wall. A pocket having a substantially smaller wall thickness than the remaining wall is formed in the region of the ultrasonic transducers and the remaining wall thickness forms the membrane of the ultrasonic transducer. This assembly, also known as clamp-in, is so-to-say an intermediate form between the fixed assembly in the inner space of the conduit and the clamp-on assembly. A relatively complicated multi-part transducer design is, however, used for this purpose. The radiating surface nevertheless remains too large for a radiation at higher frequencies.
JP 2000 337 940 A shows a further throughflow measuring apparatus in which the piezoelectric elements contact the conduit wall at the base of a bore in the conduit. The problems of a sufficiently broad radiation and of a simple transducer design are in this respect likewise not solved.
It is proposed in DE 102 48 542 A1 to attach the ultrasonic transducer directly to a functional surface that is in contact with the medium. A path alignment having a component in the flow direction is achieved by chamfering the functional surfaces and thus of the conduit. A planar, unimpeded inner conduit wall is thereby precluded.
It is therefore the object of the invention to provide an improved transducer concept for a measurement of flow speeds by means of ultrasound.